1. Field of the Invention
The present invention relates to an image formation method comprising developing an electrostatic latent image to form an image by a method such as an electrophotography method, electrostatic recording method and the like, a replenishing toner used in this method and a method of producing the same, and a carrier-containing toner cartridge.
2. Description of the Related Art
In an electrophotography method, an electrostatic latent image formed on the surface of an electrostatic latent image holding member (photorecepter) is developed with a toner containing a colorant, the resulting toner image is transferred onto an image-receiving member such as paper and the like, and this is fixed by a heat roll and the like, to give an image. On the other hand, the surface of an electrostatic latent image holding member after transfer of a toner image is generally cleaned for forming an electrostatic latent image again.
Dry developers used in such an electrophotography method are roughly classified into one-component developers which provide single use of a toner produced by compounding a colorant and the like with a bonding resin, and two-component developers obtained by mixing the above-mentioned toner with a carrier. One-component developers can be classified into magnetic one-component developers which use a magnetic powder and convey an image with a developer holding member by magnetic force and cause development, and non-magnetic one-component developers which convey an image with a developer holding member by electric charge and cause development, without using a magnetic powder.
From the later period of the nineteen eighties, the market of electrophotography has received requirements for size reduction and increase in functions using a key word, digitization, and particularly with respect to full color image quality, high grade printing, and high image quality near silver halide photography are desired. As the means for attaining high image quality, digitization treatment is essential, and as the effect of such digitization regarding image quality, an ability of carrying out complicated image processing at high speed is mentioned. By this, letters and photography images can be controlled separately, and reproducibility of both qualities is improved as compared with analogy technologies. Particularly regarding photography images, that gradation correction and color correction have become possible is a great merit, and advantages exist in the points of gradation property, definition, sharpness, color reproduction and graininess as compared with an analog method.
For output of an image, a latent image made by an optical system is required to be correctly converted into an image, the particle size of a toner is further decreasing, and there is an accelerated action intending correct reproduction. Only by decrease in the particle size of a toner, however, it is difficult to stably obtain an image of high image quality, improvements of basic properties in development, transfer and fixing properties are further important.
In the case of obtaining color images, three-color or four-color toners are piled to form images, generally. Therefore, when any of these color toners exhibits different properties from the initial properties from the standpoints of development, transfer and fixing, or a different ability from those of other colors, decrease in color reproduction, deterioration in graininess, uneven color and the like would be caused. For maintaining an image of stable high quality like the initial state even with the lapse of time, the way to stably control properties of color toners is important.
Recently, from the standpoint of increase in speed in obtaining color images (may simply be referred to as xe2x80x9ccolor speed upxe2x80x9d), there is adopted a so-called tandem development system using a plurality of xerography units composed of a developer apparatus containing a developer holding member, and of an electrostatic latent image holding member and the like, and from the standpoint of trying to reduce the size of an apparatus due to need for space saving, the sizes of the electrostatic latent image holding members are intended to be reduced. Further, there are a lot of patent applications regarding the tandem development system (Japanese Patent Application Laid-Open (JP-A) Nos. 6-35287, 6-100195, and the like).
When such a tandem development system is adopted, increase in speed of color image formation becomes easier as compared with a rotary development system, however, also in trying to obtain an image of single color such as black and the like, it is general that developer holding members of other colors also come into contact with the electrostatic latent image holding member and, simultaneously, forced to rotate toward the process direction. In such as case, a developer receives large stress, decrease in the charging ability of a developer is induced, and decrease in a developing ability and decrease in a transferring ability are easily caused, finally, leading to lowering of image quality. Further, in the tandem development system, due to restriction of space around an electrostatic latent image holding member and the size of an apparatus, the size of one developer apparatus is limited, and sufficient developer amount cannot be secured in each developer apparatus from the standpoint of space. Therefore, a developer tends to receive larger stress from apparatus-structural point of view. Consequently, deterioration of a developer occurs and the developer would be changed, this leads to remarkable increase in service cost.
As means for suppressing deterioration of a developer, JP-A No. 8-234550 discloses a technology using several kinds of replenishing toners containing carriers having different physical properties. In this technology, toner flowability and toner inter-color property and the like are influenced by change of physical properties of carriers, leading to a complicated control system, increase in the size of an apparatus, or increase in cost. JP-A No. 11-202630 discloses a technology of replenishing a replenishing toner containing a carrier having a charge amount higher than that of a carrier used in the start developer. These technologies are very effective in that the developer life is elongated, however, when image stability is taken into consideration, it is important that developer physical properties are not changed by environments and lapse of time, and it is difficult to control this change into micro level.
On the other hand, also a toner has a problem that irregularity of the form and particle size of a toner causes irregularity of the charging property of a toner, toners having excellent charging property are selectively consumed and toners having low charging property remain in a developer apparatus, to cause lowering of developing property as the whole developer, indicating selective development. When deterioration of a developer progresses due to the selective development, necessity to change a developer occurs, leading to remarkable increase in service cost. Particularly in the tandem development system, since sufficient developer amount cannot be secured in each developer apparatus from the standpoint of space, deterioration of a developer because of irregularity of the charging property of a toner progresses easily, and it is desired to improve a property to maintain a developer also from the standpoint of a toner.
Further, it is reported that toners are stirred in a developer apparatus and fine structural change on the surface of a toner occurs easily, to significantly change transferring property (JP-A No. 10-312089). Because of fine structural change of the surface of a toner, irregularity of the charging property of a toner tends to increase, resulting in promotion of the above-mentioned selective development, and the problem of decrease in developer maintaining property becomes further remarkable.
Therefore, the present invention is intended to solve the above-mentioned conventional problems and to attain the following object. Namely, an object of the present invention is to provide an image formation method which remarkably elongates the developer life and can also realize maintenance-free operation, using a tandem type image formation apparatus which corresponds to size reduction and high speed coloring, a replenishing toner used in this method and a method of producing the same, and a carrier-containing toner cartridge.
The present inventors have intensively studied, and resultantly found that it is effective to adopt a so-called trickle developing system having a replenishing system and a discharging system to replenish appropriately a replenishing toner composed of a toner and a carrier into a developer apparatus and recovering an excess portion of the above-mentioned developer from the equipment in the tandem type image formation apparatus, and to use a specific carrier or toner as the above-mentioned replenishing toner, leading to completion of the present invention.
According to a first aspect of the present invention, there is provided an image formation method of conducting image formation by an image formation apparatus having a plurality of xerography units containing an electrostatic latent image holding member; a charging means for charging the surface of the electrostatic latent image holding member; a latent image forming means for forming a latent image on the surface of the above-mentioned electrostatic latent image holding member charged; a developer apparatus accommodating a developer composed of a toner and a carrier and developing the above-mentioned latent image by a layer of the above-mentioned developer formed on the surface of the developer holding member, to form a toner image on the surface of the above-mentioned electrostatic latent image holding member; and a transferring means for transferring the above-mentioned toner image onto an image-receiving member, wherein
the developer apparatus of at least one xerography unit in the above-mentioned image formation apparatus has a developer recovering mechanism replenishing appropriately the replenishing toner composed of a toner and a carrier into the developer apparatus and recovering an excess portion of the above-mentioned developer from the equipment,
the above-mentioned replenishing toner has a carrier content in the range of 5 to 40% by weight,
the above-mentioned carrier is produced by coating a resin containing a conductive material on a core material and the above-mentioned resin for coating a core material is a copolymer composed of a monomer containing a carboxyl group, a monomer containing fluorine, a branched alkyl methacrylate monomer having 3 to 10 carbon atoms, and an alkyl methacrylate monomer containing a linear alkyl group having 1 to 3 carbon atoms and/or an alkyl acrylate monomer containing a linear alkyl group having 1 to 3 carbon atoms.
According a second aspect of the present invention, there is provided an image formation method of conducting image formation by an image formation apparatus having a plurality of xerography units containing an electrostatic latent image holding member; a charging means for charging the surface of the electrostatic latent image holding member; a latent image forming means for forming a latent image on the surface of the above-mentioned electrostatic latent image holding member charged; a developer apparatus accommodating a developer composed of a toner and a carrier and developing the above-mentioned latent image by a layer of the above-mentioned developer formed on the surface of the developer holding member, to form a toner image on the surface of the above-mentioned electrostatic latent image holding member; and a transferring means for transferring the above-mentioned toner image onto an image-receiving member, wherein
the developer apparatus of at least one xerography unit in the above-mentioned image formation apparatus has a developer recovering mechanism replenishing appropriately the replenishing toner composed of a toner and a carrier into the developer apparatus and recovering an excess portion of the above-mentioned developer from the equipment,
the above-mentioned replenishing toner has a carrier content in the range of 5 to 40% by weight,
the above-mentioned toner has a volume average particle size of 3 to 10 xcexcm and the toner shape factor SF1 of the formula (1) is from 110 to 135:
SF1=R2/Axc3x97xcfx80/4xc3x97100 
(wherein, R represents the maximum length of a toner, and A represents a projected area of a toner.).
In these image formation methods of the present invention (when simply referred to as xe2x80x9cimage formation method of the present inventionxe2x80x9d, it means both of the image formation method of the present invention according to the first aspect and the image formation method of the present invention according to the second aspect.), it is preferable that the above-mentioned xerography unit having a xerography unit contains further a means of cleaning the surface of an electrostatic latent image holding member after transfer of a toner image by the above-mentioned transferring means.
As described above, the present invention enables provision of an image excellent in image stability by using a developing system and a developer exhibiting little change over a long period of time in physical properties such as charging deterioration, resistance change and the like, in a tandem type image formation apparatus having a plurality of electrostatic latent image holding members and developer holding members and which is required to have high reliability.
Specifically, a trickle developing system is adopted, and in the present invention according to the first aspect, a copolymer obtained by combining specific monomers is used as a coating resin of a resin-coated layer of a carrier, and in the present invention according to the second aspect, a toner having a form near sphere is used. For attaining high image quality at high level, it is preferable to combine the present invention according to the first aspect and the present invention according to the second aspect.
According to the present invention of the first aspect, it is possible to provide a carrier for developing an electrostatic latent image and a developing system which are excellent in charging property under high humidity, suppress charge increase under low humidity, prevent peeling of a resin-coated layer, cause no easy adhesion of a toner and outer additives, cause no change of flowability and conveyability of developer, and excellent in maintenance property. Further, by placing a conductive material in the form of a matrix in a resin-coated layer, it is possible to form an image causing little change in resistance over a long period of time even if it receives carrierxe2x80x94carrier stress and carrier-toner stress and having high image quality.
On the other hand, according to the present invention according to the second aspect, flowability, charging property and transferring property are improved because a toner having high sphericity (near sphere) is used. Particularly, since the form of a toner is near sphere and the forms are totally uniform, irregularity of charging properties of toners is suppressed, problems owing to selective development are reduced, and the maintenance property of a developer is improved. Further, since the form of a toner is near sphere, change in fine structure of the surface of a toner is not caused easily and selective development is not promoted, even by various stresses.
Further, the image formation method of the present invention can be applied suitably to an image formation apparatus which can change process speed automatically or manually by given conditions.
In the above-mentioned xerography unit having a developer recovering mechanism, it is preferable that the above-mentioned charging means is a charging equipment of roll charging mode.
On the other hand, the replenishing toner of the present invention is characterized in that it is used in the above-mentioned image formation method of the present invention, and it is preferable to produce the replenishing toner by selecting carriers from an excess developer recovered by the above-mentioned developer recovering mechanism in the above-mentioned image formation method of the present invention, and mixing these as all or a part of carriers into a toner. In this procedure, the volume specific resistivity of all carriers mixed into a toner is preferably from 107 to 1014 xcexa9xc2x7cm.
The carrier-containing toner cartridge of the present invention is a toner cartridge for replenishing a replenishing toner into a developer apparatus of an image formation apparatus, and characterized in that it accommodates the above-mentioned replenishing toner of the present invention.